A combination of the words boob and virginity. Girls loose this when they first let another person ( guy or girl, I dont judge ) grab or feel up their chest. it is sort of like loosing your virginity but only with your chest. Also guys can loose this when they feel or touch thier first set of breasts.

Note: over the bra does not count, ( it can be related to oral sex, you are almost there but not quite. )